There are a number of operational parameters that can be of concern in oscillator operation, depending on the oscillator's intended application. In oscillators intended for measurement instrumentation applications, for example, "phase noise" is often a critical parameter. Other parameters of concern may include output power and compression characteristic. Many of these parameters are influenced by the base inductance (or gate inductance) of the oscillator transistor.
In an oscillator intended for fixed frequency operation, it is relatively straightforward to select a base inductance that optimizes parameters of particular concern. However, a serious problem is evident in oscillators that can be tuned to operated over a broad band of frequencies: what base inductance should be selected?
The usual approach is to select a fixed, compromise base inductance that permits adequate (rather than optimum) oscillator operation over a desired range of frequencies. By so doing, however, optimum oscillator performance is achieved at only one frequency, if at all. Further, the use of fixed base inductance necessarily limits the range of possible operating frequencies, sometimes preventing certain design criteria from being met.
An alternative approach is to electronically alter the topology of the base inductance network to provide different inductances at different frequencies. This can be accomplished by means such as PIN diodes that switch certain circuit elements into or out of the circuit when predetermined tune voltage thresholds are passed. This approach, however, suffers by reason of its complexity and by reason of parasitic problems associated with the additional circuit elements.
Another approach is to design a network that achieves a desired reactance vs. frequency characteristic across the band of interest. However, this approach is difficult to implement without introducing too much loss.